Carbon nanotube array and method for forming same

ABSTRACT

A method for forming a carbon nanotube array using a metal substrate includes the following steps: providing a metal substrate ( 11 ); oxidizing the metal substrate to form an oxidized layer ( 21 ) thereon; depositing a catalyst layer ( 31 ) on the oxidized layer; introducing a carbon source gas; and thus forming a carbon nanotube array ( 61 ) extending from the metal substrate. Generally, any metallic material can be used as the metal substrate. Various carbon nanotube arrays formed using various metal substrates can be incorporated into a wide variety of high power electronic device applications such as field emission devices (FEDs), electron guns, and so on. Carbon nanotubes formed using any of a variety of metal substrates are well aligned, and uniformly extend in a direction substantially perpendicular to the metal substrate.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to carbon nanotube arrays andmethods for forming them, and more particularly to carbon nanotubearrays formed on metal substrates and methods for forming said carbonnanotube arrays. This application relates to a contemporaneously filedapplication having the same title, the same applicant and the sameassignee with the instant application.

[0003] 2. Description of Prior Art

[0004] Carbon nanotubes are very small tube-shaped structures eachhaving the molecular structure of a graphite sheet rolled into a tube.Carbon nanotubes produced by arc discharge between graphite rods werefirst discovered and reported in an article by Sumio Iijima entitled“Helical Microtubules of Graphitic Carbon” (Nature, Vol. 354, Nov. 7,1991, pp. 56-58). Carbon nanotubes have excellent mechanical propertiessuch as a high Young's modulus, a high elastic modulus, and low density.Carbon nanotubes also have excellent electrical, electromechanical andabsorption properties. Carbon nanotubes display metallic properties orsemiconductor properties according to different ways in which thegraphite sheet is rolled. Due to these and other properties, it has beensuggested that carbon nanotubes can play an important role in fieldssuch as microscopic electronics, materials science, biology andchemistry.

[0005] The chemical vapor deposition method is known in the art as beingconducive to growing carbon nanotube arrays with well aligned carbonnanotubes. In the chemical vapor deposition method, a carbon source gasis thermally decomposed at a predetermined temperature in the presenceof a transition metal that acts as a catalyst, thereby forming a carbonnanotube array.

[0006] When a carbon nanotube array is used as an electrode in a fieldemission display, electron gun or other high power electronic device,the substrate on which the carbon nanotube array is formed must havegood conductivity and the ability to carry high current loads. Thus, ametal substrate is best for such applications.

[0007] However, carbon nanotube arrays are generally formed onnonmetallic substrates such as silicon, silicon oxide or glass. This isbecause metal substrates generally impair high-density formation ofpure, straight carbon nanotubes. In particular, a metal substrate isliable to react with the nanotube catalyst to form an alloy thereof,which renders the catalyst inactive. In addition, when the metalsubstrate itself is the catalyst, it is prone to decompose the carbonsource gas to form amorphous carbon. Amorphous carbon inhibits formationof carbon nanotubes of the desired quality. In summary, it is difficultto optimally form carbon nanotubes using metal substrates.

[0008] A carbon nanotube assembly and method for producing the same aredisclosed in China patent application CN1241813A. The assembly comprisesa metal substrate and a coating of carbon nanotubes deposited thereon.The method includes the following steps: (1) separating formed singlewall carbon nanotubes by chemical cutting technology; (2) dispersing theseparated carbon nanotubes in water uniformly to form a water-solublegel, and allowing the water-soluble gel to settle for an extendedperiod; (3) selecting a band of the water-soluble gel which containscarbon nanotubes having a desired uniform length; (3) diluting theselected band of water-soluble gel with deionized water; and (4) coatingthe selected carbon nanotubes on a clean metal substrate to form carbonnanotubes standing upright on the metal substrate. However, this methoddoes not provide original formation of carbon nanotubes directly on themetal substrate. In addition, when said water-soluble gel is allowed tosettle, the time required is unduly long: typically around one month.These difficulties greatly limit industrial applications of the carbonnanotube assembly and method.

[0009] A method for producing a carbon nanotube array on an aluminum(Al) substrate was reported in an article by Ch. Emmenegger et al.entitled “Carbon nanotube synthesized on metallic substrate” (AppliedSurface Science, Vol. 162-163, 2000, pp. 452-456). The method comprisesthe following steps: (1) coating iron nitrate (Fe(NO₃)₃) on an aluminumsubstrate; (2) annealing the substrate with the Fe(NO₃)₃ film to formnano-sized particles of ferric oxide (Fe₂O₃); and (3) introducing amixture of carbon source gas and protective gas until a carbon nanotubearray extends from the substrate.

[0010] However, the metallic material used as the substrate in theabove-described method by Ch. Emmenegger et al. is limited to justseveral metals such as aluminum and nickel (Ni). This limitation isnecessary to prevent the metal substrate from adversely affectingformation of the carbon nanotubes by reacting with the catalyst or bydecomposing the carbon source gas to form amorphous carbon. Accordingly,the method is generally limited to applications in which aluminum andnickel electronic devices can be utilized.

SUMMARY OF THE INVENTION

[0011] In view of the above-described drawbacks, an object of thepresent invention is to provide a method for forming a carbon nanotubearray using a metal substrate wherein the metal substrate may be any oneof a wide variety of metallic materials.

[0012] Another object of the present invention is to provide a carbonnanotube array formed using a metal substrate, wherein the metalsubstrate can be used as an electrode in a high power electronic device.

[0013] In order to achieve the objects set out above, a preferred methodof the present invention for forming a carbon nanotube array using ametal substrate comprises the following steps: providing a metalsubstrate; oxidizing the metal substrate to form an oxidized layer on asurface of the metal substrate; depositing a catalyst layer on theoxidized layer; introducing a carbon source gas; and thus forming acarbon nanotube array extending from the treated substrate. Generally,any metallic material can be used as the metal substrate. Various carbonnanotube arrays formed using various metal substrates can beincorporated into a wide variety of high power electronic deviceapplications such as field emission devices (FEDs), electron guns, andso on. Carbon nanotubes formed using any of a variety of metalsubstrates are well aligned, and uniformly extend in a directionsubstantially perpendicular to the metal substrate.

[0014] Other objects, advantages and novel features of the presentinvention will become more apparent from the following detaileddescription when taken in conjunction with the accompanying drawings, inwhich:

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 is a flowchart of a preferred method for forming a carbonnanotube array using a metal substrate according to the presentinvention;

[0016]FIG. 2 is a schematic view of a metal substrate used in thepreferred method of the present invention;

[0017]FIG. 3 is a schematic view of the substrate of FIG. 2 with anoxidized layer thereon;

[0018]FIG. 4 is a schematic view of the treated substrate of FIG. 3 witha metal catalyst layer formed on the oxidized layer;

[0019]FIG. 5 is a schematic view of the treated substrate of FIG. 4after annealing, wherein the catalyst layer has become oxidized catalystparticles; and

[0020]FIG. 6 is a schematic view of a carbon nanotube array formed onthe treated substrate of FIG. 5.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

[0021] A preferred method for forming a carbon nanotube array using ametal substrate according to the present invention will be describedwith reference to the flowchart of FIG. 1.

[0022] Referring also to FIGS. 2-6, a metal substrate 11 is firstprovided (step 10). The substrate 11 can be a metal plate or a metalelectrode formed on a base. If a base is used, it can be glass, silicon,quartz or another suitable material having a smooth surface. The metalelectrode can be formed on the base by plating or sputtering. Theselected metal plate or metal electrode is polished to obtain a smoothsurface thereon. The metal substrate 11 can be nickel (Ni), steel,tantalum (Ta), silver (Ag) or another suitable metallic material.

[0023] In principle, there is no particular limitation on selection ofthe metal plate material or the metal electrode material. Any metallicmaterial used in the semiconductor industry can be used as the metalsubstrate 11. However, there are four criteria that must be satisfiedwhen selecting the metal plate material or metal electrode material: (1)the metal plate material or metal electrode material does not melt attemperatures below chemical vapor deposition (CVD) temperatures; (2) themetal electrode material does not melt with its accompanying base attemperatures below CVD temperatures; (3) in the temperature range fromroom temperature to CVD temperature, a coefficient of thermal expansionof the metal electrode material is compatible with that of its base; (4)during formation of carbon nanotubes, the metal plate material or metalelectrode material does not expand or break if it absorbs hydrogen. Inthe preferred method, steel is used as the metal substrate 11.

[0024] The metal substrate 11 is oxidized in air at 500° C. to form anoxidized layer 21 on a surface thereof (step 20). The oxidized layer hasa thickness of several tens of nanometers. Then a metal catalyst layer31 is deposited on the surface of the oxidized layer 21 by electron beamevaporation, thermal evaporation or sputtering (step 30). The catalystlayer 31 has a thickness in the range from one nanometer to tennanometers. The catalyst layer 31 can be iron (Fe), cobalt (Co), nickel(Ni), or any suitable combination alloy thereof.

[0025] The treated metal substrate 11 is annealed in air at 300° C. forseveral hours, thereby oxidizing the catalyst layer and forming catalystparticles 31′.

[0026] A carbon source gas is then introduced for a while(step 40). Thecarbon source gas can be acetylene, ethylene, or any suitable chemicalcompound which contains carbon. In the preferred method, ethylene isused as the carbon source gas. Then the carbon nanotube array 51 isformed and extended from the treated substrate 11 (step 50). Followingthe formation of carbon nanotubes a by-product of the nanotube formationsuch as hydrogen gas is produced and reduces the oxidized layer 21 ofthe metal substrate 11. Therefore, the carbon nanotube array 61 isformed directly on the metal substrate 11.

[0027] It will be apparent to those who having skill in the field of thepresent invention that the oxidized layer 21 formed on the metalsubstrate 11 prevents the metal substrate 11 from reacting with thecatalyst, and prevents the metal substrate 11 from decomposing thecarbon source gas to form amorphous carbon. Furthermore, the oxidizedlayer 21 maintains the conditions for forming carbon nanotubes as usedin conventional CVD methods. Thus, generally, any metallic material canbe used as the metal substrate 11. Various carbon nanotube arrays 51formed on various metal substrates 11 can be used in a wide variety ofhigh power electronic device applications such as field emission devices(FEDs), electron guns, and so on.

[0028] It is understood that the invention may be embodied in otherforms without departing from the spirit thereof. Thus, the presentexamples and embodiments are to be considered in all respects asillustrative and not restrictive, and the invention is not to be limitedto the details given herein.

1. A method for forming a carbon nanotube array, comprising thefollowing steps: providing a metal substrate; oxidizing the substrate toform an oxidized layer on a surface of the substrate; depositing acatalyst layer on the oxidized layer; and introducing a carbon sourcegas; whereby the carbon nanotube array formed extends from the metalsubstrate.
 2. The method for forming a carbon nanotube array asdescribed in claim 1, wherein the substrate is a metal plate, or a metalelectrode formed on a base.
 3. The method for forming a carbon nanotubearray as described in claim 2, wherein a material of the metal substrateis selected from the group consisting of nickel, tantalum, silver andsteel.
 4. The method for forming a carbon nanotube array as described inclaim 3, wherein the material of the metal substrate is steel.
 5. Themethod for forming a carbon nanotube array as described in claim 1,wherein the substrate is oxidized in air at approximately 500° C.
 6. Themethod for forming a carbon nanotube array as described in claim 1,further comprising the following step: polishing the substrate to obtaina smooth surface thereon.
 7. The method for forming a carbon nanotubearray as described in claim 1, wherein the catalyst layer comprisesmaterial selected from the group consisting of iron, cobalt, nickel andany suitable combination alloy thereof.
 8. The method for forming acarbon nanotube array as described in claim 7, wherein the catalystlayer comprises iron, and has a thickness in a range of 1-10 nanometers.9. The method for forming a carbon nanotube array as described in claim1, further comprising the following step: annealing the treatedsubstrate in the air at a temperature of 300° C. for several hours. 10.The method for forming a carbon nanotube array as described in claim 1,wherein the carbon source gas is selected from the group consisting ofacetylene, ethylene and any suitable chemical compound which containscarbon.
 11. The method for producing a carbon nanotube array asdescribed in claim 10, wherein the carbon source gas is ethylene.
 12. Acarbon nanotube array subassembly comprising: a metal substrate; and acarbon nanotube array formed thereon; wherein a material of the metalsubstrate is selected from the group consisting of tantalum, nickel,silver and steel.
 13. A carbon nanotube array subassembly comprising: abase provided with a metallic upper face; said upper face being oxidizedto form an oxidized layer thereon; a catalyst layer applied on saidoxidized layer; and a carbon nanotube array growing on said catalystlayer.